Fluorescent lamp dimming circuit



Oct. 3, 1961 D. D. KERSHAW FLUORESCENT LAMP DIMMING CIRCUIT 5 Sheets-Sheet 1 Filed Nov. 8, 1954 Inventor: Delmar D. Kegshaw. b 7 5 y His A orney Oct. 3, 1961 D. D. KERSHAW FLUORESCENT LAMP DIMMING CIRCUIT Filed NOV. 8, 1954 3 Sheets-Sheet 2 ifi- 2 la. lb ld F 'j l l I l 1 I lj gj U n I /5I 5a L 1 Q 325 I 35 Inventor: Delmar D. Kehshaw.

by (T His Attorney Oct. 3, 1961 D. D. KERSHAW 3,003,079

FLUORESCENT LAMP DIMMING CIRCUIT Filed Nov. 8, 1954 3 Sheets-Sheet 3 Li 4-! :2; B z 2 a :5 42 520 Z50 o 23 O. u: In 2 if z 550 I- a .J d. '1 2' 40 43 300 1 l--- 4 o N d] 0 Ba 0 '7. ROTATION OF CONTROL REACTOR Inven 3,003,079 FLUORESCENT LAMP DIMMING CIRCUIT Delmar D. Kershaw, Highland Heights Village, Ohio, as-

signor to General Electric Company, a corporation of New York Filed Nov. 8, 1954, Ser. No. 467,349 9 Claims. (Cl. 315-97) This invention relates in general to circuits for varying the luminous output of electric discharge lamps. It relates particularly to a dimming circuit for fluorescent lamps operated from an alternating current supply of substantially constant voltage. V

The invention is applicable to discharge lamps of the kind having filamentary electrodes designed to be continuously supplied with'heating current during the operation of the lamp. The commercially available fluorescent lamps of this kind are commonly known as rapid start lamps. They are characterized by filamentary activated electrodes of small size and some means extending the length of the lamp which facilitates the establishment of a potential gradient for starting the lamp at a low voltage. This means may take the form of a Water repellent coating on the lamp envelope which cooperates with a conductive member to assure reliable starting under substantially any atmospheric conditions. In the case of the common 40-watt rapid start fluorescent lamp of 48-inch nominal length, the requirement of a conductive member extending the length of the lamp is generally economically fulfilled by the metal channel or housing of the fixture. The fixture is grounded whereby it is placed at a definite potential difference with respect to the high" electrode of the lamp, that is, the electrode connected to the side of the operating circuit which is at a high voltage relative to ground.

The fluorescent lamp dimming circuits and devices which have been available up to the present have in general been either too expensive or too unreliable to meet the needs of a mass market such as the home lighting field. It will readily be appreciated that a dimming circuit comprising anumber of thyratron control tubes, whereas it may be entirely suitable for stage or auditorium lighting, is generally out of the question for residential use by reason of its cost. A dimming control unit for residential use does not have to meet the stringent specifications usually applicable to stage or auditorium lighting. However, even for residential use, it is necessary that the dimming control dim all the lamps connected to it equally and that all the lamps ordinarily.

startreliably at any setting of the control unit.

Accordingly, the object of the invention is to provide a new and improved circuit for varying the luminous output of electric discharge lamps.

Another object of the invention is to provide a dimming control system which is low in cost and reliable in operation.

Yet another object of the invention is to provide a dimming control system for discharge lamps having electrodes designed for continuous heating which permits even dimming of a plurality of lamps from a single control and which assures reliable starting of all the lamps at any setting of the control.

In accordance with the invention, the lamp is provided with a ballast transformer having a primary winding energized from the usual alternating current supply lines, a secondary winding having sufficient leakage reactance to limit the lamp current to its rated value when the secondary is connected in autotransformer relation with the primary across the lamps, and auxiliary windings providing the'voltage for heating the electrodes of' nited States Pater reactor, variable from zero to full impedance, is con-= nected in series into the lamp circuit, preferably by insertion between the high side of the primary winding and the low side of the secondary winding in the ballast transformer. Where several lamps are to be dimmed together, each lamp is provided with a ballast transformer, as described, and the control reactor is serially inserted in common with all the lamp circuits. In other words, the high sides of the primary windings in all the ballasts are connected to one common point, and the low sides of all the secondary windings are connected to another common point, and the single control reactor is connected between these two common points. In this manner, the control reactor provides a common variable impedance in series with the ballast secondary reactances andn the lamps, and allows the lamp current in each circult to be equally divided and varied in a uniform manner.

For further objects and advantages and for a better understanding of the invention, attention is now directed to the following description and accompanying drawings. The features of the invention believed to be novel will be more particularly pointed out in the appended claims.

In the drawings:

FIG. 1 illustrates, in part diagrammatically, a dimming control system in accordance with the invention applied to a single discharge lamp;

FIG. 2 illustrates schematically a dimming control system in accordance with the invention for a plurality of discharge lamps;

FIG. 3 illustrates graphically the operating characteristics of a typical dimming control system in accordance with the invention;

FIG. 4 illustrates schematically another dimming control circuit applied to a single lamp.

' Referring to FIG, 1, the illustrated dimming control system varies the luminous output of an electric discharge lamp \1. The lamp may be a 40-watt rapid start fluorescent lamp comprising an elongated tubular envelope 2 having sealed into its ends a pair of filamentary activated electrodes 3, 4 which are of a small size and heated to electron emission at voltages below the ionization voltage of the gas or vapor within the lamp. The electrodes are of the coiled-coil type provided with an overwind holding a relatively large quantity of activated material comprising alkaline-earth oxides. The filling within the envelope comprises a starting gas, such as argon, at a pressure of a few millimeters of mercury and a small quantity of mercury. The envelope is coated interiorly with a phosphor which converts the ultraviolet radiation produced by the discharge into visible light. The exterior of the envelope is coated with a waterrepellent substance which assures reliable low voltage starting under any atmospheric conditions when the lamp is placed close to a conducting member maintained at a suitable potential difference with respect to. one of the electrodes. Such member is here illustrated by the elongated conductive plate 5 located close to the lamp envelope and connected to ground. In the usual commercial installation, the function of member 5 is performed by the metal reflector plate of the grounded fixture, this plate being ordinarily spaced about inch from the envelope wall.

The lamp is operated by means of the ballast transformer 6 illustrated within the dotted rectangle. This unit may be similar to commercially available low power factor ballasts for operating a single 40-watt rapid start fluorescent lamp. The transformer comprises a magnetic core formed of electrical steel laminations in the usual manner. The windings are mounted upon a central winding leg 7 on either side of which are butted E-shapedj laminations 8, 9 with magnetic shuntsll and air gaps 12 between primary and secondary windows 13, 14. Primary winding 15 along with auxiliary electrode heating windings 16, 17 are wound about central leg 7 and located in window 13, whereas secondary winding 18 is similarly wound about leg 7 and located in window 14, This arrangement provides tight coupling between primary and auxiliary windings whereby to maintain the electrode heating current constant, and loose coupling between primary and secondary windings whereby to achieve high leakage reactance for regulating the current through the lamp.

Auxiliary windings 16, 17 are connected across electrodes 3, 4 by conductors 19, 20 and 21, 22, respectively. Conductor 19 also connects to the low voltage end of primary winding 15, whereas conductor 21 connects to the high voltage end of secondary winding 18, these connections being such as to place auxiliary windings 16, 17' in aiding voltage relationship to primary winding 15 and secondary winding 18, respectively. Primary Winding 15 is connected across the usual 115-120 volt, 60- cycle commercial supply indicated by conductors 23, 24, conductor 23 being the grounded or low side of the supply as indicated by the dotted ground connection at 25. The low voltage end of primary winding 15 is connected to grounded supply conductor 23 through conductor 19 and conductor 26. The connection from conductor 26 to conductor 19 is elfected through lamp socket 27 which is of the well-known threeterrninal disconnect type as diagrammaticallyindicated. This mode of connection insures that the primary voltage is removed from the transformer whenever the lamp is disengaged from its socket, this feature serving to reduce personnel shock hazard. Socket 28 at the other end of the lamp may be an ordinary two-terminal socket. The high voltage end of primary winding .15 is connected directly to the high side of the voltage supply, namely conductor 24, by means of conductor 29.

In accordance with the invention, dimming of the lamp is eifected by serially connecting a variable control reactor or inductor 31 between the high voltage end of the primary winding and the low voltage end of the secondary winding of the ballast =autotransformer. As here illustrated, reactor 31 comprises an annular core 32 formed by winding a strip or ribbon of electrical steel into a tight spool. Winding 33 is ring wound around core 33 with one end connected by conductor 34 to the high side of the voltage supply, namely conductor 24, thereby eflecting a connection to the high voltage end of primary winding 15. The low voltage end of secondary winding 18 is connected by means of conductor 35 to a rotatable contact arm 36 which engages the turns of winding 33 by means of -a brush 37, such as a carbon block. Clockwise rotation of contact arm 36 increases the proportion of control reactor 31 serially included in the secondary circuit of the ballast transformer and progressively dims the lamp. Throughout most of the dimming range except for settings near full dimming, the core operates at saturation.

The arrangement according to the invention, wherein the variable reactor is inserted between the high side of the primary and the low side of the secondary, makes it possible to place disconnect socket 27 in the primary circuit at the lamp terminals. Thus all secondary voltages are removed whenever the lamp is removed from its socket. This feature of the invention is an important one because, in the absence thereof, the necessary safety feature would have to be provided at considerably higher cost, for instance by means of an insulated switch built into the lamp socket with attendant additional wiring.

A very important feature of the invention is that the circuit of FIG. 1 may be expanded to achieve dimming control of a large number of lamps from a single control reactor 31. Each lamp in the group is connected, as illustrated in FIG. 1, across its individual ballast transformer 6. Such an arrangement is illustrated in FIG. 2

for the case of a four-lamp dimming control system. Lamps 1a to 1d are connected across ballast transformers 6a to 6d, respectively. The primary windings of the ballast transformers are connected by conductors 26, 29 across supply conductors 23, .24, respectively, conductor 24 serving as a common point for the high voltage ends of the primary windings in the ballasts. The low voltage ends of the secondary windings are connected to control conductor 35 which serves as the other common point. Control reactor 31 is connected between the two common points, that is, between conductors 24 and 35.

Among the desirable characteristics of a dimming control system are that lamps must start reliably at any point in the dimming range, dimming must be smooth and free of lamp flicker, and it must permit switching on or off any number of lamps without affecting brightness or stability of those remaining in the system. Furthermore, the system should be capable of reducing lamp brightness to a few percent of maximum, preferably down to l percent. These desirable characteristics are realized by the system illustrated in FIGS. 1 and 2. Actual systems have been constructed and tested for groups of 4, 8 and 13 lamps. The performance characteristics of an S-lamp system are illustrated graphically in FIG. 3. Referring to FIG. 3, solid line curve 41 represents the percent light output of the lamp plotted as ordinate against percent rotation of the control reactor plotted as abscissa. It will be observed that the luminous output decreases almost linearly with rotation of the control reactor. Dash line 42 represents the lamp current (total for all 8 lamps) under the same conditions. Dot-dash line 43 represents the peak starting voltage available to the last lamp in the group to start for any setting of the control reactor. This quantity was determined by removing one lamp from the group and measuring the voltage available at its sockets after all 7 remaining lamps have started. In order to insure reliable starting of 40-watt rapid start fluorescent lamps, a peak voltage of 250 volts is ordinarily sufficient. It will be observed that the lowest peak starting voltage provided by the system and which occurs at percent rotation of the control reactor, that is at the minimum brightness position, has a value of 270 volts. This is well above the nominal peak voltage required, so that reliable starting of all the lamps is obtained.

For the convenience of those desiring to practice the invention, essential illustrative particulars of a dimming control system for eight 40-watt rapid start fluorescent lamps in accordance with the invention are given below, reference being made to FIGS. 1 and 2:

The core of ballast transformer 6 consists of .025 inch thick electrical steel laminations stacked to a height of 4%; inch. Central winding leg 7 is .75 inch wide and 4.20 inches long. The width of the long portions of E-shaped members 8, 9 is .375 inch; the air gaps 12 are .020 inch. The core in FIG. 1 has been drawn to scale, so that the other dimensions may readily be computed by sealing from the drawing. Primary winding 15 consists of 773 turns of .0142 inch copper wire; auxiliary windings 16, 17 each consist of 26 turns of .0142 inch copper wire wound over the primary; secondary winding 18 consists of 778 turns of .0126 inch copper wire. Capacitor 45 shown in FIG. 1 is provided for radio interference suppression and has a value of .05 microfarad. Resistor 46 establishes a connection from the low side of the supply to the ballast case and therethrough to the fixture; it has a value of 1 megohm.

Core 32 of control reactor 31 has an internal diameter of 1 /2 inches, an external diameter of 3% inches and a height of 1 /2 inches. The core is drilled through with five radial holes 47 of approximately V inch diameter in order to effect a more gradual saturation in the core at maximum percentage rotation; the holes are located one above the other so that only one appears in dotted outline in 'FIG. 1. Winding 33 consists of 350 turns of No. 27 copper wire, ring wound around 330 degrees of the core. This unit is similar to commercially available variable autotransformers having a rating of 115 to 120 volts, 50 to 60 cycles, with a maximum current output of 3 amperes. I

As the number of lamps included in a dimming control system with a given reactor 31 is increased, it may be found that the reactor core comes out of saturation too rapidly at the maximum rotation end of the control reactor. This is evidenced by a rapid flattening of the lower end of curves 41 and 42, as shown in FIG. 3, and results in too abrupt a change in current and luminous output. This may be corrected by placing an auxiliary-inductance 48 in shunt with control reactor 31; as illustrated in FIG. 2, this may be effected by closing switch 49. The shunt reactance may be used to provide a higher starting voltage for all the lamps in the 100 percent dimming setting of the control reactor. It also assures a higher re-ignition voltage for the lamps whereby to achieve greater operating stability and to equalize the light output from the various lamps, despite variations in their individual electrical characteristics. It will be appreciated that the electrical characteristics resulting from the provision of reactor 48 could be achieved by choice of iron and core structure in reactor 51 alone.

FIG. 4 illustrates schematically another embodiment of the invention wherein the low power factor ballast 6 of FIG. 1 is replaced by a high power factor ballast 51. Ballast 51 may include a core similar to that of ballast 6 and including primary winding 15 and auxiliary windings 16, 17 tightly coupled together, and secondary winding 18 loosely coupled to the primary. In addition, primary winding 15' has an extended winding 52 tightly coupled thereto, and a capacitor 53 is connected across the ends of the primary and extended winding. Capacitor 53 draws a leading current from the primary which offsets the lagging magnetizing current, whereby to provide a high over-all power factor. The provision of extended winding 52 serves to reduce the size of capacitor required. Ballast 51 is in other respects similar to ballast 6 and it is similarly connected into the dimming control system with control reactor 31.

Essential illustrative particulars of a high power factor ballast, such as shown in FIG. 4, are as follows:

The core is similar to that which has been described earlier by reference to FIG. 1: the primary winding, the auxiliary windings, and the extended winding are all wound about the central winding leg and located within the larger Window; the secondary winding is similarly wound and located in the smaller window. Primary winding 15 consists of 712 turns of .0089 inch copper wire; auxiliary windings 16, 17 each consist of 31 turns of .0126 inch copper wire; auxiliary winding 18 consists of 778 turns of .0126 inch copper wire and extended winding 52 may consist of 1346 turns of .0089 inch wire. Capacitor 53 may have a value of 2.0 microfarads with a voltage rating of 350 volts A.C. The ballast is in other respects similar to that described earlier.

While certain specific embodiments of the invention have been illustrated and described in detail, these are intended as illustrative and not as limitative examples of the invention. Various modifications in the values and proportions and ratings of the component elements will readily occur to those skilled in the art in order to adapt the system to specific operating conditions. The appended claims are intended to cover any such modifications coming within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A dimming control system comprising an elongated electric discharge lamp of the low pressure positive column type having a pair of thermionic filamentary activated electrodes sealed into opposite ends thereof, an alternating current supply of constant voltage, a ballast transformer comprising a magnetic core, a primary winding connected across said supply, a pair of low voltage auxiliary-windings connected across respective ones of said electrodes, and a secondary winding loosely coupled to said primary winding to provide a high leakage reactance therebetween, said primary winding and said secondary winding being serially connected in autotransformer relation across opposite electrodes of said lamp, and a variable inductive reactance serially inserted between the high voltage end of said primary winding and the low voltage end of said secondary winding.

2. A dimming control system comprising an elongated electric discharge lamp of the low pressure positive column type having a pair of thermionic filamentary activated electrodes sealed into opposite ends thereof, an alternating current source of substantially constant voltage, a. ballast transformer comprising a magnetic core, a primary winding connected across said source, a pair of auxiliary low voltage windings tightly coupled to said primary winding and connected across respective ones of said electrodes, a secondary winding loosely coupled to said primary winding with high leakage reactance therebetween, a variable inductive reactance, and connections from the low voltage end of said primary winding to one electrode of said lamp, from the high voltage end of said secondary winding to the other electrode of said lamp, from the high voltage end of said primary winding to one side of said variable inductance, and from the low voltage end of said secondary winding to the other side of said variable inductance.

3. A dimming control system comprising a plurality of elongated electric discharge lamps of the low pressure positive column type having a pair of thermionic filamentary activated electrodes sealed into opposite ends thereof, an alternating current supply of substantially constant voltage, a plurality of ballast transformers, one for each of said lamps, each of said ballast transformers comprising a magnetic core, a primary winding connected across said supply, a pair of low voltage auxiliary windings respectively connected across opposite electrodes of the associated lamp, and a secondary winding loosely coupled to said primary winding with high leakage reactance therebetween, the low voltage end of the primary winding and the high voltage end of the secondary winding in each ballast being connected to opposite electrodes of the associated lamp, the high voltage ends of the primary windings in all the ballasts being connected to one common point, the low voltage ends of the secondary windings in all the ballasts being connected to another common point, and a variable inductive control reactor connected between said two common points.

4. A dimming control system comprising a plurality of elongated electric discharge lamps of the low pressure positive column type having a pair of thermionic filamentary activated electrodes sealed into opposite ends thereof, an alternating current supply of substantially constant voltage, a pair of sockets for each lamp includmg one socket of the disconnect type, a plurality of ballast transformers, one for each of said lamps, each of said ballast transformers comprising a magnetic core, a primary winding connected across said supply through the disconnect socket of the associated lamp, a pair of low voltage auxiliary windings respectively connected across opposite electrodes of the associated lamp, and a secondary winding loosely coupled to said primary winding with high leakage reactance therebetween, the low voltage end of the primary winding and the high voltage end of the secondary winding in each ballast being connected to opposite electrodes of the associated lamp, the high voltage ends of the primary windings in all the ballasts being connected to one common point, the low voltage ends of the secondary windings in all the ballasts being connected to another common point, and a variable inductive control reactor connected between said two common points.

5. A dimming control system comprising a plurality of elongated electric discharge lamps of the low pressure 7 v positive column type having a pair of thermionic filamentary activated electrodes sealed into opposite ends thereof, an alternating current supply of substantially constant voltage, a plurality of ballast transformers, one for each of said lamps, each of said ballast transformers comprising a magnetic core, a primary winding connected across said supply, an extended winding tightly coupled to said primary winding and connected in series therewith, a capacitor connected across the series combination of said primary and extended windings for eifecting power factor correction, a pair of low voltage auxiliary windings respectively connected across opposite electrodes of the associated lamp, and a secondary winding loosely coupled to said primary winding with high leakage reactance therebetween, the low voltage end of the primary winding and the high voltage end of the secondary winding in each ballast being connected to opposite electrodes of the associated lamp, the high voltage ends of the primary windings in all the ballasts being connected to one common point, the low voltage ends of the secondary windings in all the ballasts being connected to another common point, and a variable inductive control reactor connected between said two common points.

6. A dimming control system comprising a plurality of elongated electric discharge lamps of the low pressure positive column type having a pair of therminoic filamentary activated electrodes sealed into opposite. ends thereof, an alternating current supply of substantially constant voltage, a plurality .of ballast transformers, one for each of said lamps, each of said ballast transformers comprising a magnetic core, a primary winding connected across said supply, a pair of low voltage auxiliary windings respectively connected across opposite electrodes of the associated lamp, and a secondary winding loosely coupled to said primary winding with high leakage reactance therebetween, the low voltage end of the primary winding and the high voltage end of the secondary winding in each ballast being connected to opposite electrodes of the associated lamp, the high voltage ends of the primary windings in all the ballasts being connected to one common point, the low voltage ends of the secondary windings in all the ballasts being connected to another common point, and a variable inductive control reactor connected between said two common points, said reactor comprising a saturable magnetic core, a winding thereon, and means for varying the proportion of the winding connected in circuit.

7. A control system as defined in claim 6 including in addition a fixed inductance connected in parallel with the variable reactor for reducing the abruptness of change from saturated to nonsaturated condition in the core thereof.

8. An adjustable intensity lighting circuit comprising at least one fluorescent lamp having a pair of filaments, ballast means for continually heating the filaments and providing an impedance during operation of the lamp, and adjustable reactor means including a winding wound on a magnetic core and a tap operable on the winding for adjusting the arc current flow between the filaments and hence the intensity of the lamp.

9. An adjustable intensity lighting circuit comprising more than one fluorescent lamp, with each lamp having a pair of filaments; ballast means for each lamp for continually heating the filaments and including an impedance means operable during operation of the lamp to limit the arc current; circuit means connecting the ballasts in parallel to a source of electrical energy; and a single adjustable reactor connected in the circuit to be in series with each impedance means for adjusting the arc current flow between the filaments.

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